The invention relates to a device for contactless, inductive power and data transmission between at least one writing and reading unit with an antenna coil, and at least one mobile data memory, which is preferably fitted to a portable object, with an antenna coil. The principle of a contactless transmission path of this type corresponds to a transformer with a coupling factor which varies as a function of distance.
In the case of such devices, which are frequently referred to as identity systems, a mobile data memory is first of all supplied with power in a contactless manner by means of a writing and reading unit for the purposes of activation and for enabling a data exchange. This is done inductively in that a magnetic alternating field is generated in the surrounding space by an antenna coil belonging to the writing and reading unit. When an object having a mobile data memory fitted thereon enters the magnetic alternating field, a voltage is induced in the antenna coil thereof. This provides the power required to operate the mobile data memory. The data transmission which is now possible between the writing and reading unit and the mobile data memory also takes place inductively via the two antenna coils. Information identifying the type and the current condition of the object provided with the respective mobile data memory is, as a rule, communicated as data to the writing and reading unit for a wide variety of purposes.
The maximum distance between the mobile data memory and the writing and reading unit is, as a rule, up to one meter and, in practice, depends on the dimensions of the antenna coils of the writing and reading unit and the mobile data memory and on the respective ambient conditions. Particular problems arise when electrically conductive materials are present in the region of the alternating field, since the eddy currents induced therein cause the antenna resonant circuits of the writing and reading unit and/or of the mobile data memory to be detuned and/or the magnetic alternating field to be completely shielded from the antenna coil of the mobile data memory. As a result, the maximum range of the data transmission can be restricted right up to the point of complete interruption of said data transmission. A particularly unfavorable situation is when a metallic surface of an object must serve as support for a mobile data memory.
To solve this problem, it is admittedly possible to fit a mobile data memory by means of special mounts in such a way that an adequate distance from the electrically conductive surroundings or support is maintained. This ensures that the inductive power and data transmission is influenced as little as possible by the otherwise occurring resonant frequency shifts in the antenna resonant circuits of the mobile data memory or the writing and reading unit. However, it is disadvantageous that the mobile data memories project sometimes to a large extent over the outer surfaces of the objects and, consequently, there is a considerable risk of breaking. Furthermore, mounts protruding in this way will make it more difficult to transport and use, in particular, objects which are difficult to handle on account of their shape, for example, and, if appropriate, have a high dead weight.
If a mobile data memory is fastened to a preferably portable object to be identified, then data transmission without additional positioning operations is only possible if the sum of the minimum distance to be maintained between the mobile data memory and the writing and reading unit and the largest edge lengths thereof is always less than the range of the device under the respective ambient conditions. Only in simple cases, however, is it ensured that the mobile data memory, when manually or automatically approaching the writing and reading unit, is always situated in the field region thereof in a manner independent of position. Further problems thus arise in the case of parallelepipedal objects having relatively large dimensions or in the case of objects having curved outer surfaces. In this case, prior positioning is frequently required for an automated data exchange, for example, where an object that is provided with a mobile data memory travels on a conveyor belt past a stationary writing and reading unit. It is therefore necessary, under certain circumstances, that the object be rotated until a mobile data memory, situated on a remote side in the most unfavorable case, is facing the writing and reading unit. This is particularly difficult with objects which have a high dead weight or curved outer surface and the mobile data memories consequently have to be fitted on poorly definable, non-planar surfaces.
Further problems arise when a plurality of objects, each provided with a mobile data memory, are simultaneously brought up, for example on a pallet, in a box or other transport device, to a writing and reading unit. On the one hand, particularly when the objects have, for example, a cylindrical shape, the mobile data memories on some objects may randomly assume a position facing away from the respective writing and reading unit. A type of shading thus occurs, which is particularly pronounced in the case of objects produced from a metallic material. On the other hand, in conventional identity systems, as a rule only one mobile data memory is allowed to be situated in the magnetic alternating field of the writing and reading unit during the data exchange, in order to maintain proper functioning. Certain identity systems generate large magnetic fields within which the data exchange between a mobile unit and a writing and reading unit occurs. The relatively large magnetic fields produced by such systems permit the data exchange to occur largely independent of the relative positioning of the mobile unit and the writing and reading unit. Nevertheless, this advantage is nullified as soon as a plurality of objects are situated at a small distance from one another, for example in a common transport container. Due to this simultaneous presence of a plurality of mobile data memories in the field region of a writing and reading unit, it is no longer possible, without a (possibly considerable) additional software and hardware outlay, to set up a data transmission to each individual mobile data memory in an unambiguously distinguishable manner.